EAE is induced because of inflammation of central nervous system (CNS) primarily caused by autoreactive CD4+ T cells. It is well documented that myelin-specific T cells, especially TH1 cells and more recently TH17 cells, are important in the initiation of tissue inflammation in EAE. We have demonstrated that IL-27, a member of the IL-12 family of cytokines, directly suppresses the induction of TH17 cells and generates regulatory IL-10-producing type 1 regulatory T (Tr1) cells. However, the mechanism by which IL-27 mediates inhibition of TH17 cells and promotion of Tr1cells is not well understood. In vivo administration of IL-27 suppresses the development of Th17 cells and EAE. Our preliminary data suggests that IL-27 controls tissue inflammation in EAE by two different but interlinked mechanisms: 1, by inducing the expression of IL-12R2, and therefore inhibiting the generation of Th17 cells in an IL-12 dependent manner; 2, by enhancing the expression of two transcription factors, AhR and c-Maf, that generate Tr1 cells. In this project, we will molecularly define the mechanism by which IL-27 regulates tissue inflammation in EAE by studying the following specific aims: 1) Mechanism by which IL-27 inhibits generation of Th17 cells by studying whether induction of IL-12R2 chain by IL-27 not only suppresses IL-23R expression but also makes Th17 cells highly responsive to IL-12 and make them plastic to produce IFN-y; 2) How IL-27 generates Tr1 cells by studying the role of transcription factors AhR and cMaf and their mutual association in differentiating Tr1 cells. These studies will be accomplished by using AhR, cMaf and AhR x cMaf double knock-out mice; 3) Study interplay between Th17 and Tri in the development of tissue inflammation in EAE, using IL-17, IL-10 and FoxP3 reporter mice. These studies will provide a mechanistic basis of how IL-27 suppresses autoimmunity and tissue inflammation in the CNS.